The music player is the standard Android one given a blue-tinged redesign, so it's pretty basic. You're able to organise files by folder or playlist, and Sony Ericsson has shoehorned its Infinite button into the mix.
Pressing that performs an internet search for the artist you're currently listening to, bringing up a list of YouTube videos that the phone thinks you might want to watch.
It works best if you're listening to something modern and popular that there's lots about on the internet, but even then it's only really a novelty option for the extremely bored, as YouTube videos on the Xperia X10 Mini Pro aren't really up to much.
Music playback is great through the supplied headphones, and the phone's speaker is so loud you could stick the X10 Mini Pro on the bonnet of a parked car and use it to start a rave. It's going to be the cause of many a fight on public transport systems, we suspect.
The YouTube app is extremely impressive, playing back files completely glitch-free and skipping back and forth through videos in a fraction of a second, as long as you're connected through Wi-Fi. However the small screen isn't the ideal vehicle to be viewing them through.
Outside of Google's safely-controlled video portal, media playback is a little more restrictive. The X10 Mini Pro requires files to be encoded into MP4 format in order to play, so you can forget about seamlessly copying across your existing AVI collection without substantial re-encoding hell.
Once on the phone in the correct format, though, video files play and zip about as well as those within the YouTube app. It really is quite amazing to see a phone so small performing so admirably.